Vehicle interior panels, such as door panels, dashboard panels, and headliner panels, are often formed out of a thermoplastic sheet. A show surface of the thermoplastic sheet (i.e., the exposed surface of the eventual panel that can be observed by a vehicle occupant) is sometimes grained and tipped to enhance the appearance of the surface, and in some cases to furnish surface qualities like softness and matte. In a known process, a coating is applied to the show surface of the thermoplastic sheet and to raised portions of pre-grained individual granules (a so-called tipping process). The coating can be a gloss, paint, or other material.
In some known applications, the now grained and coated thermoplastic sheet is formed to its product shape by a thermoforming process using a positive or male tool. The thermoplastic sheet is heated and brought into direct contact with an ungrained outer surface of the positive tool. In some cases, the details of the grain are lost, or washed out of the thermoplastic sheet, by the heating and stretching involved in the thermoforming process with the positive tool, and what are left are grains more elongated and more rounded than desired. Also, the stretching caused by the positive tool—especially stretching at sharper radii features and edges—can cause the previously imparted grains to elongate and become distorted.
Negative graining of a pre-textured panel is also known. U.S. Pat. No. 6,841,105 to Nehring et al. discloses texturing thermoplastic panels on a surface thereof, and disposing the pre-textured surface against a textured mold surface.